This invention relates to a light modulator that comprises an optically anisotropic medium capable of exhibiting an electrooptic effect. Such a light modulator is useful in modulating, among others, the phase, the polarization, and/or the intensity of a laser beam with a modulating signal of a frequency between zero and about 1 GHz.
A light modulator is indispensable in most systems in which a laser beam is used in transmitting, recording, reading out, or displaying an enormous amount of information at a high speed. Examples of such systems are optical communication systems, optical memory systems, laser recording systems, and laser display systems. On modulating a light beam that may not necessarily be a coherent beam, a light modulator makes use of mechanical deformation or vibration, an acoustooptic effect, or an electrooptic effect. Among such light modulators, an electrooptic light modulator has the desirable features of being operable in a wide frequency band, at a high speed, and with a high conversion efficiency.
An electrooptic light modulator comprises a piece of crystal that is transparent to a light beam to be modulated and exhibits an electrooptic effect. The crystal is preferably either a uniaxial or a biaxial crystal. It is possible, as the case may be, to use a bulk of an optical isotropic material that shows optical anisotropy when subjected to pressure and/or the like. In a first type of the electrooptic light modulator, a spatially periodic electric field is applied to the crystal by the modulating signal so as to make the crystal have a phase lattice for subjecting an incident light beam to Bragg diffraction or Raman-Nath diffraction. In a second type, the difference in phase delays between a pair of incident light components polarized in the directions of two principal axes of an optical indicatrix (index spheroid or ellipsoid) of the crystal is electrically controlled by the modulating signal so as to modulate the polarization of the incident light. In a third type, a spatially periodic electric field is distributed along light propagating direction, and an optical indicatrix of the crystal is rotated by the modulating signal so as to convert at least a portion of a linearly polarized component of an incident light beam to a cross-polarized component in an output light beam as will be detailed later with reference to a few figures of the accompanying drawing. A light modulator of the third type may be called a phase-matched electrooptic light modulator because two orthogonally polarized components resulting from the linearly polarized component of the incident beam are matched in phase during propagation through the crystal as will become clear as the description proceeds.
A light modulator of the first type comprises electrodes arranged on a principal surface of the crystal in a predetermined pattern for producing the spatially periodic electric field in the crystal. The electrode pattern gives rise to spurious diffraction. It is difficult to attain a large diffraction angle. It is therefore impossible to achieve a high on-off ratio that corresponds to the extinction ratio to be described in the following.
The crystal for use in a light modulator of the second type should have a large electrooptic constant. Examples are an ammonium dihydrogen phosphate (ADP) crystal, a potassium dihydrogen phosphate (KDP) crystal, and a lithium methatantalate (LiTaO.sub.3, usually referred to as lithium tantalate) crystal. The ADP and the KDP crystals are defective in that they diliquescence and require a high modulating voltage. A lithium tantalate crystal does not deliquescence and is operable at a low modulating voltage. A high extinction ratio, however, is not attainable by the use of only one lithium tantalate crystal because the cross-polarized component appears in the output light beam as an unavoidable result of a shift in the ambient temperature from a predetermined temperature even when no modulating voltage is applied to the crystal. The extinction ratio may be raised with a pair of crystals cascade-connected in compensation for the temperature shift. It is, however, difficult with such a composite light modulator to achieve a sufficient temperature stability due to the difficulties in obtaining homogeneous crystals and in manufacturing the composite light modulator with high precision. Furthermore, a restriction is imposed on the wavelength of the incident light.
A phase-matched electrooptic light modulator is operable with a relatively low modulating voltage and provides a high extinction ratio. The conversion efficiency of a sophisticated phase-matched electrooptic light modulator, however, decreases according as the ambient temperature shifts from a predetermined temperature at which the phases of the orthogonally polarized components are matched. This results in a narrow temperature range in which the light modulator is operable. In other words, the temperature at which the light modulator is kept in operation has to be fairly strictly controlled. An improved light modulator of this type is disclosed in Japanese Publication of Unexamined Patent Application (Tokkyo Kokai Koho) No. Syo 54-27455 of 1979 by the present applicant. According to the improvement, the operable temperature range is widened with the difference between two principal indices of refraction substantially continuously varied along the direction of propagation of the light beam being modulated. Although the conversion efficiency is kept high in a wide temperature range, the conversion efficiency still considerably fluctuates within the operable temperature range. Another improved light modulator of this type is revealed, prior to the above-mentioned improvement by the applicant, in Japanese Publication of Unexamined Patent Application No. Syo 53-93856 in 1978 by Yoshinori Ohta, assignor to the instant assignee. This prior improvement is similar to the later improvement as regards the fluctuation of the conversion efficiency. A change in the wavelength of the incident light beam also results in a similar undesirable fluctuation in the conversion efficiency.